Altered Ig Hypermutation Pattern and Frequency in Complementary Mouse Models of DNA Polymerase z Activity

To test the hypothesis that DNA polymerase z participates in Ig hypermutation, we generated two mouse models of Pol z function: a B cell-specific conditional knockout and a knock-in strain with a Pol z mutagenesis-enhancing mutation. Pol z-deficient B cells had a reduction in mutation frequency at Ig loci in the spleen and in Peyer's patches, whereas knock-in mice with a mutagenic Pol z displayed a marked increase in mutation frequency in Peyer's patches, revealing a pattern that was similar to mutations in yeast strains with a homologous mutation in the gene encoding the catalytic subunit of Pol z. Combined, these data are best explained by a direct role for DNA polymerase z in Ig hypermutation. T he genes encoding the Ag-interacting portions of Abs and IgRs are subjected to a process of deliberate hyper-mutation during immune responses leading to enhanced affinity of Abs to a specific Ag (1, 2). This mechanism, termed somatic hypermutation (SHM), is triggered by the activation-induced deaminase (AID), a molecule expressed when B lym-phocytes are activated by foreign Ag (3, 4). AID deaminates cytosines in the DNA encoding the Ig variable (V) regions (5). Mice and humans defective in AID lack SHM and class switch recombination (CSR), as AID is also required to generate switched Abs such as IgG, IgA, and so on (3, 4). A subset of hyper IgM syndrome patients are defective in AID; these patients lack CSR and SHM, and they suffer from lymph node hyperplasia (6). Despite the fact that AID is a cytosine deaminase, mice deficient in AID lack mutations at both G-C and A:T bp (3). This paradox is in part explained by the hypothesis that AID-mediated deamina-tion of cytosines in Ig V regions triggers the recruitment of translesion synthesis DNA polymerases to Ig loci (7). These DNA polymerases have relaxed geometric requirements and thus are prone to inserting incorrect bases during DNA synthesis (8). One such polymerase is DNA polymerase h, which has been shown to play a role in the misinsertion of bases at A:T sites during SHM (9, 10). Other DNA polymerases have also been implicated in SHM, but because they have important roles in other cellular functions such as cell division and DNA repair, discerning whether they play a direct or indirect role in SHM has been difficult (11–13). The mutations made during SHM of Ig V genes are predominantly base substitutions. The pattern of …